1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets, and an image forming apparatus equipped with the same.
2. Related Background Art
A sheet feeding apparatus connected to or equipped in an image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus is generally so constructed that plural sheets stacked on a sheet stack tray are elevated to a predetermined position, and are separated and fed one by one from the uppermost one for example by a feed roller.
Also a widely utilized apparatus employs a DC motor as a power source for elevating the sheet stack tray, and is equipped with height detection means for the sheet stack tray or the sheets, for detecting the height of the sheet stack tray or the uppermost sheet stacked thereon, in order to maintain the sheet stack tray at an optimum height relative to the feeding roller.
There is also known, as disclosed in Japanese Patent Application Laid-open No. H10-231029, an apparatus employing a stepping motor as the power source for the sheet stack tray and equipped with height detection means for detecting the height of the sheet stack tray, and adapted to vary a current supplied to the stepping motor as the power source or a revolution thereof by means of a detection signal of the height detection means.
In such prior structures, however, when an inexpensive DC motor is employed as the power source, the number rotations of such DC motor varies as shown in FIG. 6, depending on an amount (weight) of the sheets stacked on the sheet stack tray.
FIG. 6 shows a number of rotations of the DC motor per unit time (hereinafter simply referred to as “rotation number”) in the ordinate, as a function of a load torque of the DC motor corresponding to a sheet stacking amount in the abscissa.
According to FIG. 6, the rotation number of the DC motor increases as the sheet stacking amount on the sheet stack tray decreases.
Therefore, in case a small amount of sheets is stacked on the sheet stack tray, when the sheet stack tray is elevated and the DC motor is stopped after detecting that the uppermost sheet reaches the predetermined height, the uppermost sheet on the sheet stack tray is elevated to a position higher than the predetermined feed position. Stated differently, the sheet stack tray causes an overrun in case a small amount of sheets is stacked thereon.
This is because the rotation number of the DC motor increases for a small sheet stack amount, and, as a result, the height of the stacked sheets becomes higher than the predetermined height for sheet feeding.
In such situation where the uppermost sheet is stopped at a position higher than the predetermined feed position, the sheet may impinge on the feed roller at the feeding operation, thereby causing a jam and hindering an appropriate sheet feeding operation.
In order to suppress a fluctuation in the overrun of the sheet stack tray, Japanese Patent Application Laid-open No. H03-23120 discloses a constitution in which the movement of the sheet stack tray is switched up, down, up and down at timings when the output of a detection sensor for detecting the uppermost sheet position is turned off, on, off and on. In such constitution, since the once elevated sheet stack tray is then lowered and then elevated again, there is required a longer time until the sheet stack tray is stopped at the predetermined position and becomes ready for sheet feeding.
On the other hand, an apparatus utilizing a stepping motor for the power source is costly because the stepping motor is more expensive in comparison with the DC motor.